Add thread-safe video access

[gitttt-1234]

This PR updates the VideoReader and LabelReader classes to ensure thread-safe video access. Backend video handles are now closed in the main thread and reopened only within the worker thread that loads images. After predictions are complete, the videos are reopened in the main thread so the frames can be accessed again during tracking.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.49%. Comparing base (ff91433) to head (2527060).
⚠️ Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
- Coverage   95.28%   93.49%   -1.80%     
==========================================
  Files          49       49              
  Lines        6765     6974     +209     
==========================================
+ Hits         6446     6520      +74     
- Misses        319      454     +135     

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